Ethylene and propylene are important fundamental raw materials for petrochemical products. Ethylene and propylene are mainly produced by thermally cracking paraffin-based hydrocarbons, such as natural gases, naphthas, and gas oils, at a high temperature of 800° C. or more, in the presence of steam. To increase the yield of olefins such as ethylene and propylene in the steam cracking of hydrocarbons, a higher conversion rate of hydrocarbons or a higher selectivity of olefins are required. However, such a steam cracking process alone has limitations on increasing the conversion rate of hydrocarbons or the selectivity of olefins. In this regard,various methods capable of increasing the yield of olefins have been suggested.
As a method for increasing the yield of ethylene and propylene by the steam cracking of hydrocarbons, a catalytic steam cracking process has been suggested.
There are known a catalyst including magnesium oxide and zirconium oxide (U.S. Pat. No. 3,644,557), a catalyst consisting essentially of calcium aluminate (U.S. Pat. No. 3,969,542), a zirconia oxide supported manganese oxide catalyst (U.S. Pat. No. 4,111,793), a magnesium oxide supported iron catalyst (European Patent No. 0212320 A2), and a method for producing olefins using a catalyst including barium oxide, alumina, and silica (U.S. Pat. No. 5,600,051). However, these catalysts have a common problem in that excessive catalyst coking occurs during steam cracking of hydrocarbons.
Decomposition of hydrocarbons at a high temperature generates excessive cokes. To remove such cokes, steam is used as a diluent for reactants. However, cokes are still excessively generated and accumulated on reactor walls, which causes various problems. That is, cokes deposited on a wall surface of a cracking tube increase a thermal transfer resistance, thereby decreasing the amount of thermal transfer to hydrocarbons. Furthermore, the increase of a thermal transfer resistance leads. to further heating of a reactor to supply a sufficient thermal energy necessary for cracking reaction, which increases the surface temperature of the reactor, thereby reducing the expected life span of the reactor. In addition, the cokes deposited the wall surface of the reactor decrease an effective sectional area of the reactor, thereby increasing a differential pressure of the reactor. As a result, more energy for compressing and supplying reactants is required.
As described above, cokes generated in steam cracking of hydrocarbons increase a thermal transfer resistance and a differential pressure, which inhibits normal operation of a reactor. For normal operation of a reactor, the operation of the reactor should be suspended to remove cokes. In particular, when a catalyst is used in steam cracking of hydrocarbons, cokes are accumulated not only on a wall surface of a reactor but also on a catalyst surface. In this regard, a coking problem may worsen in the case of using a catalyst in steam cracking of hydrocarbons. Cokes accumulated on a catalyst surface lowers the performance of the catalyst, and at the same time, rapidly increase a differential pressure applied to a catalyst bed. For this reason, the operation of a reactor should be more frequently suspended for normal operation of the reactor. A catalyst surface serves to capture and condense cokes precursors formed in a vapor phase. Some catalyst components have activity facilitating cokes formation. In this regard, a catalyst for hydrocarbon steam cracking must have a property capable of maximally preventing coking.
With respect to commercially available steam cracking reactors, generally, cokes removal is performed at an interval of 30-60 days. For this, the operation of the reactor is suspended and cokes are burned off with air blowing under a steam atmosphere. A time required for cokes removal varies according to the amount of cokes accumulated in a reactor. Generally, cokes removal takes 1-2 days. However, a use of a catalyst with poor cokes removal performance may significantly increase the number of cokes removals. Even though a use of such a catalyst may increase the yield of ethylene and propylene, the production of ethylene and propylene per unit time may be reduced, relative to that in a simple cracking process. Furthermore, additional costs for cokes removal may increase significantly. In this regard, to perform a catalytic steam cracking process of hydrocarbons in a cost effective manner, a catalyst capable of decreasing the number of cokes removals by minimization of catalyst coking is required.
U.S. Pat. No. 3,872,179 discloses a zirconia catalyst containing alkaline metal oxide and Russian Patent No. 1,011,236 discloses a boron oxide-grafted potassium vanadate catalyst supported on an alumina carrier. These patents relate to cokes removal via gasification to reduce coking on a surface of a catalyst. The alkaline metal oxide and potassium vanadate serve to convert cokes to COx via gasification, which is very effective in cokes removal. Furthermore, an increase of a loading amount of the catalysts enhances the performance of cokes removal and decreases the number of cokes removals. However, the alkaline metal oxide and potassium vanadate may be present in a liquid phase in a hot cracker due to their low melting point. Therefore, during fast flow of reaction gases, catalyst components may be dissipated by volatilization with time, thereby decreasing the lifespan of a catalyst. To compensate for such a catalyst loss, addition of catalyst components during cracking reaction is necessary.